1. Field of the Invention
The subject invention generally pertains to locating couplings (or collars) within a wellbore, and more specifically to distinguishing between the couplings on the internal and external piping.
2. Description of Related Art
Many wellbores consist of a pipe within a pipe. The outer one is generally referred to as an "outer casing" or "production string" and is typically made up of 40 to 45 foot sections coupled together by way of "casing collars." The inner one, of 30 to 33 foot lengths, is called an "inner pipe string," and it is interconnected by "pipe couplings." It becomes necessary to both locate and distinguish which couplings belong to which string of pipe deep within a wellbore for the purpose of logging, perforating, or disassembly and repair.
With existing coupling sensors, it can be difficult to distinguish between casing collars and pipe couplings. Some sensors may have external hardware that properly orients the sensor in relation to the inner pipe string. However, the hardware often hangs up within the limited space of a well. Space is especially limited in multiple string wells, such as the one illustrated in U.S. Pat. No. 3,064,571 which is specifically incorporated by reference herein.
An even greater problem exists when attempting to perforate the outer casing of a wellbore. To effectively perforate a well casing, it is critical to have the face of the perforating gun up against the inner wall of the casing. Otherwise, the stand-off distance between the face of the gun and the casing wall becomes too great. Exceeding the design stand-off distance even slightly can reduce the gun's explosive impact force to a level so low that the gun fails to perforate the casing. Magnets and other mechanisms can be used to urge the gun to its proper orientation. But when those fail, one can be misled to believe that complete perforation was accomplished, because the gun still discharges. It is difficult to determine whether any perforation occurred.